The U.S. should prepare for a cyber attack that could cause destruction on the scale of 9/11 in less than 15 minutes, a leading anti-terrorism expert has warned.

Richard Clarke, a former adviser to both Bill Clinton and George W. Bush, suggests that the lack of security in place against such an attack could lead to an ‘electronic Pearl Harbor’.

Writing in his new book Cyber War: The Next National Security Threat, penned with Robert Knake, Mr Clarke says: ‘The biggest secret about cyber war may be that at the very same time the U.S. prepares for offensive cyber war, it is continuing policies that make it impossible to defend effectively from cyber attack.’